(1) Field of the Invention
The present invention relates to a ferroelectric liquid crystal display device with each pixel divided into sub-pixels and more particularly relates to a method of displaying gradations in the apparatus.
(2) Description of the Prior Art
As a method of displaying tonal gradations in a liquid crystal display device using a ferroelectric liquid crystal, a pixel dividing method is disclosed in Japanese Patent Application Laid-Open Hei 2 No.96118 in which pixels are each composed of a multiple number of sub-pixels using plural scanning electrodes and plural signal electrodes for each pixel so that each sub-pixel can individually be driven whereby gradations are displayed for each pixel. According to this method, it is possible, as shown in FIG. 1, to prevent neighboring pixels from forming unintentional pairing because the center of each pixel with divided sub-pixels is fixed. In the figure, reference numerals (1) through (16) designate display tonal level numbers for pixels having different on-and-off patterns.
In the ferroelectric liquid crystal display device thus divided, every scan line constituting sub-pixels is provided with a driver circuit so that the driver circuits sequentially sweep the scan electrodes for each pixel. In this case, time required for one pixel to be rewritten becomes long as the number of the scan electrodes for the pixel increases. For example, as comparing the time between the case where no pixel division is made and the case where one pixel is constituted by two scan lines, the latter requires two times longer than the former.
On the other hand, a method is disclosed in Japanese Patent Application Laid-Open Hei 3 No.189622 in which a plurality of scan electrodes are connected to one common driver circuit via different resistors in order to reduce the number of driver circuits. In this method, a multiple number of the scan electrodes can simultaneously be selected and consequently, the number of the scan electrodes increases and this solves the problem that the time required for one pixel to be rewritten becomes long. In this method, ITO electrodes are used as transmission lines in a way. That is, as the resistance of a transmission line becomes great, the phase of an input waveform is delayed more. Therefore, an input applied voltage to one pixel presents different waveforms between at a sub-pixel on a scan electrode connected to a driver via a low resistance and at a sub-pixel on another scan electrode connected to the driver via a high resistance even if the sub-pixels belong to the same pixel. As a result, the display states of the sub-pixels vary depending upon the state of the waveform applied to the signal electrodes. Specifically, both the sub-pixels may be rewritten, either of them may be rewritten, or neither of them may be rewritten.
Since the resistivity of ITO electrodes is typically rather high, a signal provided even for the sub-pixels on the scan electrode which are connected to the driver via an ITO electrode having a relatively low resistance delays in phase, especially at the distally located sub-pixels. Since the signal or waveform appearing on a sub-pixel located on the input side on a scan electrode must be more delayed in phase than the voltage waveform applied to the distally located sub-pixel in order to obtain uniformly gradational display on the panel, the waveform applied to distally located sub-pixels on the scan electrode connected to the driver via the high resistance must further delay in phase. Accordingly, in order to allow the sub-pixel receiving the most delayed waveform in phase to display, a longer scanning time is required for the case of this method as compared to the case where two tones or white and black tones are displayed.
Japanese Patent Application Laid-Open Sho 64 No.61180 discloses a method of displaying in which 2.sup.K tones are displayed by time-dividing using pixels which are merely able to effect binary display. In this method, all the scan electrodes for a liquid crystal display device are divided into a plurality of sets, each of which is scanned K-times in the duration of one frame. For example, if 2.sup.3 levels of tones are to be displayed, all the scan electrodes are divided into a pair of sets. As shown in (1) and (2) in FIG. 2, data on bit 1 is displayed in the first set and then data on bit 2 is displayed in the first set, data on bit 2 is displayed in the second set, data on bit 3 is displayed in the first set, data on bit 1 is displayed in the second set and data on bit 3 is displayed in the second set, whereby eight levels of tones are displayed.